Insulin-like growth factor- I (IGF-I) is a ubiquitously expressed 7.5 kDa polypedtide that circulate in relatively high concentrations. In bone, IGF-I is secreted by osteoblasts, and is stored in the matrix bound to IGF binding proteins (IGFBPs). Skeletal IGF-I promotes osteoblast differentiation, increasing collagen biosynthesis and mildly enhancing bone cell mitogenesis. Reduced serum IGF-I concentrations have been linked to lower skeletal levels of this peptide, future risk of hip fracture, histomorphometric indices of bone formation, bone mineral density, and a poly-morphism in the IGF-I gene. Hence serum IGF-I has been considered an intermediate skeletal phenotype. In the first three years of our proposal, using inbred strains of mice have established: 1) serum IGF-I is a heritable polygenic train; 2) three major non-growth hormone dependent quantitative trait loci (QTLs) and one interactive QTL influence the serum IGF-I phenotype; 3) two IGF-I QTLs associate with major skeletal phenotypes (BMD and femoral length); 4) one QTL contains the IGF-I gene and the interactive QTL maps in close proximity to the IGFBP-3 gene; 5) congenic mice carrying one of these major QTLs exhibit reduced serum IGF-I and a major skeletal phenotype; and 6) osteoblasts from high serum IGF-I mice (C3H/HeJ) show a sixfold up-regulation in IGF-I exon 1 promoter mRNA compared to the low serum IGF-I mice (C57BL/6). These data suggest that IGF-I is important for the acquisition of BMD and optimal strength. In this proposal we hypothesize that serum IGF-I genes are also major determinants of skeletal IGF-I expression. Our goals are to fine map these IGF-I regulatory genes and to understand how these genes affect osteoblast function and bone morphology. We propose three specific aims: 1) to delineate the genes for serum IGF-I by fine mapping and assessing nested congenics for changes in BMD and strength; 2) to determine the role of IGFBPs in modulating skeletal IGF-I by studying osteoblast expression and secretion of IGFBPs and by phenotype 'rescue' of newly developed congenics through targeted over expression of skeletal IGF-I; and 3) to determine the molecular mechanisms responsible for interstrain differences in skeletal IGF-I expression. These experiments will provide tremendous insight into the cellular regulation of the IGFs and IGFBPs, an important first stem towards understanding the role of IGF-I in osteoporosis and other chronic disease wherein tissue specific IGF-I activity may have a major pathophysiologic component.